The present invention relates, in general, to hearing aids and, in particular, to a hearing aid having a speaker that is mounted rigidly to the hearing aid housing and an energy absorbing tip that absorbs mechanical and acoustic energy from the speaker that might otherwise create undesired feedback.
FIG. 1 is a schematic sectional view of a conventional hearing aid. This hearing aid includes a housing 10 having a sound inlet 12 extending through a faceplate 13 of the housing and through which outside sound reaches a microphone 14. The microphone develops an electrical signal representative of the outside sound and the microphone signal is conducted to an amplifier 16 that amplifies the microphone signal. Amplifier 16 is powered by a battery 18 that is held in a battery holder 20 mounted for pivotal movement about a hinge 22 on housing 10. Battery holder 20 is held in place by a battery snap 24 on housing 10.
The amplified microphone signal is conducted to a speaker 26, commonly referred to as a receiver when used in a hearing aid, that develops an audio output of the amplified microphone signal. The audio output is conducted through a speaker sound outlet 28 and a housing outlet 30, defined by a tube 32, into the ear canal of the person wearing the hearing aid.
Speaker 26 is held in a resilient suspension 34 that is mounted to housing 10 by tube 32. Tube 32 is generally flexible but is sufficiently stiff to support resilient suspension 34. As shown in FIG. 1, resilient suspension 34 is spaced from housing 10 by spaces 36 and 38 that surround the resilient suspension. Resilient suspension 34, tube 32 and spaces 36 and 38 serve to isolate vibrations generated by speaker 26 when the speaker is driven to create the audio output.
One of the constraints of existing hearing aid designs, such as the one shown in FIG. 1, is that of internal feedback. A hearing aid is designed with: (a) a microphone that picks up sound and converts the sound to an electrical signal, (b) an amplifier that amplifies the microphone electrical signal, and (c) a speaker that converts the amplified microphone electrical signal back to sound energy.
Internal feedback occurs when the sound energy generated by the speaker reaches the microphone at an intensity greater than that of the sound that was originally picked up by the microphone. The feedback path can be acoustic or mechanical or both acoustic and mechanical.
Acoustic feedback is most often the result of sound generated by speaker 26 passing through tube 32 and into housing 10 and finding a path through faceplate 13 back to microphone inlet 14. Because most hearing aid batteries require air to generate electricity, faceplate 13 is designed with openings that allow air to pass through and, as a consequence, sound also can pass through.
Mechanical feedback occurs because speaker 26 vibrates as it produces sound. These vibrations can travel through tube 32 to housing 10 or through resilient suspension 34 to housing 10 when the resilient suspension is in contact with the housing. In practice, it is difficult to maintain spaces 36 and 38 between resilient suspension 34 and housing 10. The designers of hearing aids strive to make hearing aids as small as possible so that the hearing aids can fit into the ear as far as possible. The mechanical vibrations conducted from resilient suspension 34 to housing 10 travel though the walls of the housing and ultimately cause a microphone 14 to vibrate. Movement of the diaphragm of microphone 14 relative to the wall of housing 10 causes microphone 14 to generate an electrical signal. Although designed to minimize the effect of such vibrations, microphone 14 cannot differentiate whether a movement is mechanical or acoustic.
Mechanical/acoustic feedback occurs when vibrations of speaker 26 cause vibration in housing 10. Some of these vibrations, in turn, generate sound energy that is picked up by microphone 14.
Selection of appropriate materials for tube 32 and resilient suspension 34 is important to prevent feedback. High durometer materials are good sound containers but transmit mechanical energy. Low durometer materials can isolate vibrations. However, low durometer materials are poor sound containers. If resilient suspension 34 makes contact with housing 10, mechanical energy can pass through. A thicker suspension helps but adds size. Allowing a greater space between resilient suspension 34 and housing 10 helps but also adds size. The designers of hearing aids constantly are faced with tradeoffs between physical size of the hearing aid and amplification by the hearing aid.
A hearing aid, constructed in accordance with the present invention, includes a housing having a sound inlet through which an outside sound enters the housing and a sound outlet through which an amplified version of the outside sound exits the housing. This hearing aid also has a battery within the housing and a microphone within the housing responsive to the outside sound entering the housing through the sound inlet for developing an electrical signal representative of the outside sound entering the housing. A hearing aid, constructed in accordance with the present invention, further includes an amplifier powered by the battery and responsive to the electrical signal for developing an amplified version of the electrical signal, an energy absorbing tip, and a speaker surrounded by the energy absorbing tip, responsive to the amplified version of the electrical signal for developing and conducting to the sound outlet the amplified version of the outside sound entering the housing, and rigidly mounted to the housing to channel acoustic energy and mechanical energy generated by the speaker to the energy absorbing tip.
According to another aspect of the present invention, the open end of the sound outlet from which sound emanates is remote from that end of the hearing aid that is innermost in the ear and closest to the ear canal of the person wearing the hearing aid.